BRE Global Discuss Evacuation Modelling and Human Behaviour in Fire
The development of numerical models to predict evacuation started in the 1980’s and was well established by the 90’s. Early models concentrated on physical aspects such as speed of movement and flow rates through various constrictions. As research has improved our understanding, later models have incorporated various aspects of human behaviour that may be as significant, if not more so, than the movement process.
Extensive research programmes and experimental evacuation studies into human behaviour within fire incidents has given a unique knowledge and understanding of factors such as alarm systems, fire safety management, occupancy type and building complexity, and the control of occupant escape behaviour, something which is important for the development of effective evacuation strategies. Research also has been undertaken to define the capabilities of disabled people, who might have one or more of a range of disabilities: physical, sensory or mental, and to different degrees of severity. They may often move slowly, and in some cases find particular obstacles difficult to pass or impassable. This knowledge base is then used to develop methods for the quantification and modelling of human behaviour during emergencies, which have been incorporated into engineering calculation and computational tools. In turn, this has led to a greater understanding of the impact on the provision of physical means of escape, which constitutes one of the greatest restraints and cost elements in building design and construction.
Safe means of escape in fire and other emergencies is a principal requirement for the design and operation of buildings and transport systems. The reasons for time taken for individuals to recognise the existence of a fire are complex and, once the existence of a fire has been recognised as a threat, people undertake a wide range of possible behaviours, some that may appear irrational. For example, they may finish tasks they were undertaking or look for family members. And rather than look for a more convenient or safer route, they often attempt to evacuate via the entrance that they used to enter their surroundings.
Evacuation time is conventionally split into ‘pre-movement’ time and ‘travel’ time. The pre-movement time incorporates the time to recognise that an alarm has been given and that some action needs to be taken, as well as time of response for all other activities performed prior to evacuation. The travel time refers to the time needed, once movement towards an exit has begun, for all occupants of a specific part of a building to reach a place of safety.
People tend to use familiar routes when trying to escape. However, there are many factors that influence a choice of exit. Congestion at an exit may act as a deterrent to further people joining the crowd, groups may stay together and all use the same exit route, visitors to the building may follow others in the hope that at least one person knows where they are going. A good design will ensure that the fire exits are part of the normal circulation routes.
Figure 1 - Computer simulations of response phases showing 'detect', 'investigate', alert others' and 'evacuate' (clockwise-from-top-to-left)
Numerical models of the evacuation process vary widely in their degrees of sophistication. The simplest is to treat the occupants as a homogeneous fluid, or mindless particles, and concentrate on the flow capacity of the building. At the other extreme there are detailed simulations where each person is treated individually, with explicit behavioural rules.
Accurate prediction of human behaviour involves the appropriate:
- Selection of a model for the scenario(s)
- Provision of model inputs for the scenario(s)
- Calculations by the model chosen
- Interpretation of the results of the model calculations by the user
For many fire safety engineering designs, the objective of the design is to maintain a clear means of escape for as long as necessary.
Before attempting modelling, it is essential to understand the phenomena being modelled. In the case of evacuation and human behaviour in a fire, some aspects, such as movement speeds, can be quantified with reasonable accuracy for most common situations. However, other aspects are much more uncertain.
BRE Global has developed two egress simulations; GridFlow and CRISP. Both models treat each person individually and use a fine network to regulate the movement process.
GridFlow has a simplified model of human behaviour, where all the "pre-movement" activities are represented by a single time delay (which may differ for each person). When these activities are complete, people move to the outside, via the nearest, the preferred, or a randomly-chosen exit. Despite this simplified behaviour, there are many situations where the model can predict the required safe egress time (RSET) with reasonable accuracy.
CRISP is a simulation of entire fire scenarios, developed as a risk assessment tool using Monte-Carlo (random number) methods. However, it can also be run in a stand-alone evacuation mode, without simulating the fire or calculating toxic exposures. The behaviour of the occupants is much more detailed than in GridFlow, making the CRISP model applicable to a wider range of scenarios.
In England and Wales, the UK Regulatory Reform (Fire Safety) Order 2005 states that the responsible person must ensure a safe, conspicuous and effective escape route is available. Such models may be of use to the manager of a complex building.
BRE Global’s recent Digest titled “Evacuation modelling and human behaviour in fire” provides practical advice and guidance on the use of numerical models to predict human behaviour during emergency building evacuation.
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